Move browser discovery into wptrunner

[gsnedders]

At the moment, we have a mixture of logic in tools/wpt/browser.py and tools/wptrunner/wptrunner/webdriver_server.py and tools/wptrunner/wptrunner/browser/*.py; we should simplify all this.

We have:

• ways to find a binary in multiple places (I'm not sure what wptrunner itself even does in some cases, and I can't figure out how to run it in isolation)
• browser_version passed in as a kwarg to wptrunner.start from wpt.run, but doesn't get passed in in any other way (everything else that ends up in kwargs comes from the argument parser)

On the face of it, what we want in wpt is just the installation management side of things.

[jgraham]

I don't think wptrunner is doing anything complex; it just takes the binary you pass it through command line options and uses that. The only change is can make is wrapping it in a debugger if you are trying to run under gdb or similar. Is that not the case?

I changed the browser_version thing as part of #12679

That said I agree with the conceptual separation that wpt run should deal with setting up the environment, and wptrunner should use the provided environment.